The present invention relates to method and apparatus for detecting a shortcircuit in an arm of an inverter, and more particularly to a method and apparatus for detecting a shortcircuit in an arm of an inverter formed by gate turn-off thyristors (GTO).
When a shortcircuit occurs in an arm of the inverter such that a P-line GTO and an N-line GTO turn on simultaneously, the current gradually increases because the GTO's are not able to function to suppress the current, resulting in breakdown of the GTO's. In the past, methods have been proposed to detect the shortcircuit in the arm of the inverter to stop the operation of the inverter device before the GTO's are broken down. In one method, a D.C. current which is a direct amount to be sensed is detected and in another method a voltage drop which is an indirect amount to be sensed is detected to sense the occurrence of the shortcircuit. Those methods, however, have the following disadvantages.
In the D.C. current detection method, when an output voltage is attenuated to zero in the GTO inverter of PWM (pulse width modulation) control type, three phases are operated in phase. Accordingly, three-phase charging currents for snubber capacitor flow coincidently in the D.C. portion of the inverter for each switching. Since these charging currents reach a large magnitude which is considerably higher than the controllable current of the GTO's, a low detection level for the shortcircuit current cannot be established due to operational margin required. That is, the detection level has to be set to a high level when the operational margin is taken into consideration. By this is meant that the detection of the shortcircuit becomes difficult accordingly. Thus, unless the resistance of the GTO to dv/dt materially increases to increase the characteristic impedance of the snubber charging circuit to decrease the charging current, the D.C. current detection method is not appropriate to detect the shortcircuit in the GTO inverter. Particularly when a shunt resistor is used for the detection, the shunt resistance increases as the inverter capacity increases and hence this method is not apropriate in view of the circuit loss and the resistance to noise.
When the shortcircuit is to be detected by the GTO voltage which is an indirect sample, it is necessary to discriminate a pseudo shortcircuit which exists in normal operation from a true shortcircuit. The time required for the discrimination is approximately 20.mu. seconds, which is fairly long. Accordingly, this method has a disadvantage in that it cannot attain high speed detection which is the most important requirement for detecting the shortcircuit. In addition, this method uses dividing resistors to detect the voltage drop of the GTO. Since a voltage is necessarily applied to the GTO for one-half period, the capacity of the dividing resistors must comply with that voltage.